As to display devices such as liquid crystal display devices, it has been known that luminance enhancement can be achieved by providing a circularly-polarized light separating sheet having a function of allowing certain circularly-polarized light to pass therethrough and reflecting other species of light, i.e., the function of selective reflection. As such a circularly-polarized light separating sheet, those taking advantage of a selective reflection due to Bragg reflection of a substance in a state of a cholesteric liquid crystal phase have been known. Specifically, there have been known those obtained by applying, orienting and polymerizing a liquid crystal composition containing a polymerizable monomer exhibiting a cholesteric liquid crystal property on a substrate, as well as those obtained by applying and orienting a polymer exhibiting a cholesteric liquid crystal property or a solution thereof on a substrate.
In terms of enhancing the display's quality, it is preferable that the bandwidth of the selective reflection of such a circularly-polarized light separating sheet is as broad as possible within the visible light range, preferably throughout the visible light range, in order to enhance the performance of the display device. As a method for obtaining such a circularly-polarized light separating sheet exhibiting a broad selective reflection bandwidth, a variety of methods have been known. As a promising method among them, there has been known a method in which a liquid crystal compound is polymerized so as to cause a gradient in the polymerization degree of the liquid crystal compound monomer along a thickness direction of the layer, thereby providing a gradient in an orientation pitch (see Patent Document 1: JP 2003-139953-A (corresponding to US Patent Application Publication No. 2003/090617)).
However, such a method requires a very strict control of the polymerization degree in the thickness direction of the layer, which is an extremely short distance. If there is even a little inappropriateness in controlling the polymerization degree, the polymerization degree may become homogeneous over the entire layer, resulting in failure in creating the gradient in the polymerization degree, ending up with a narrowed selective reflection bandwidth of the obtained circularly-polarized light separating sheet. Also in such a method, due to a slight difference of conditions in a production environment, the orientation pitch may be greatly altered. Thus, it is very difficult to form a layer having a desired pitch length and steady gradient of the pitch.